Abstract
The scope and potential uses of time-domain terahertz imaging and spectroscopy are mainly limited by the low optical-to-terahertz conversion efficiency of photoconductive terahertz sources. State-of-the-art photoconductive sources utilize short-carrier-lifetime semiconductors to recombine carriers that cannot contribute to efficient terahertz generation and cause additional thermal dissipation. Here, we present a novel photoconductive terahertz source that offers a significantly higher efficiency compared with terahertz sources fabricated on short-carrier-lifetime substrates. The key innovative feature of this source is the tight three-dimensional confinement of the optical pump beam around the terahertz nanoantennas that are used as radiating elements. This is achieved by means of a nanocavity formed by plasmonic structures and a distributed Bragg reflector. Consequently, almost all of the photo-generated carriers can be routed to the terahertz nanoantennas within a sub-picosecond time-scale. This results in a very strong, ultrafast current that drives the nanoantennas to produce broadband terahertz radiation. We experimentally demonstrate that this terahertz source can generate 4 mW pulsed terahertz radiation under an optical pump power of 720 mW over the 0.1–4 THz frequency range. This is the highest reported power level for terahertz radiation from a photoconductive terahertz source, representing more than an order of magnitude of enhancement in the optical-to-terahertz conversion efficiency compared with state-of-the-art photoconductive terahertz sources fabricated on short-carrier-lifetime substrates.
Highlights
The main problem facing such photoconductive sources is the limited number of photo-generated carriers that can effectively contribute to the pulsed terahertz radiation
We present a novel photoconductive terahertz source that overcomes the slow carrier problem by means of a nanocavity that induces the three-dimensional confinement of the optical pump light around the terahertz radiating elements
The polarization of the optical beam is set normal to the nanoantennas, and the spot size of the beam is adjusted to focus on the area covered by the plasmonic nanoantennas
Summary
The scope and potential uses of time-domain terahertz imaging and spectroscopy are mainly limited by the low optical-to-terahertz conversion efficiency of photoconductive terahertz sources. The key innovative feature of this source is the tight three-dimensional confinement of the optical pump beam around the terahertz nanoantennas that are used as radiating elements This is achieved by means of a nanocavity formed by plasmonic structures and a distributed Bragg reflector. It consists of a two-dimensional array of plasmonic nanoantennas fabricated on the active high-mobility semiconductor layer These plasmonic nanoantennas are designed to excite surface plasmon waves at the optical pump wavelengths, while serving as radiating elements at terahertz frequencies[28]. If a bias voltage is simultaneously applied, almost all of the photo-generated carriers can reach the nanoantennas and contribute to generating an ultrafast current As this strong, ultrafast current drives the terahertz nanoantennas, a much more efficient optical-to-terahertz conversion occurs compared to the state-of-the-art sources based on short-carrier-lifetime substrates, and a powerful terahertz pulse can be generated. The device fabrication process is described in the methods section
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